This application relates to the art of switches and more particularly, to thermal switches that automatically move from one position to another in response to an elevated temperature. The invention is particularly applicable to a normally closed switch that moves to an open position for preventing thermal runaway in batteries and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects and can be used in normally open switches as well as for other purposes.
Overcharging batteries of certain types may cause thermal runaway that raises the internal battery temperature. The internal pressure within the sealed battery case increases with increasing temperature and an explosion may occur when the internal pressure exceeds the bursting strength of the battery case. A battery explosion releases by-products that are caustic, toxic and flammable, and the force of the explosion may cause injury to persons and damage to property.
Thermal protectors of various types have been proposed and used in batteries for interrupting the battery circuit when a predetermined temperature is exceeded. The space available for small rechargeable batteries in modern electronic devices such as cellular telephones requires miniature temperature protection devices. Miniaturization is limited in thermal protectors that use bimetals or thermistors having a positive temperature coefficient of resistance because such devices do not operate properly or reliably if they are made too small.
Arrangements have been proposed for incorporating a thermal switch actuator of shape memory alloy into the physical structure of a battery internally of the battery case. Such arrangements cannot be used with existing battery designs and it would be desirable to have a self-contained thermal switch assembly with a current carrying actuator of shape memory alloy that could be used for a variety of purposes including external attachment to a battery case.